Known switched-mode power supplies typically use a power transistor switch, in order to feed a pulsed current flow to a network of inductive-energy and capacitive-energy storage elements that convert the switched current pulse into regulated DC voltage. According to the operating mode of the switched-mode power supply, switched-mode power supplies can supply output voltages that are larger, smaller, equal to, or of opposite polarity than the unregulated input voltage. Switched-mode power supplies are often used in AC line-powered power supply circuits. Here, such switched-mode power supplies should accept input voltages in the range of 85 V to 270 V AC and in this way should be able to operate with different AC power line distribution systems anywhere in the world without modification or switches. In addition, the output voltage must be regulated sufficiently and precisely, in order to avoid, on the one hand, damage to the load and, on the other hand, unnecessary power consumption. Finally, modern switched-mode power supplies must be extremely economical and must have physical dimensions that are minimized as much as possible.
The output voltage of known primary switched switched-mode power supplies is typically regulated by means of a feedback signal that indicates the output voltage and/or the output current. In order to be able to use the output voltage as a control parameter, the feedback of a corresponding signal must be realized from the secondary side to the control loop on the primary side. This feedback signal is used to control the operating cycle of the switching power transistor. In order to prepare a suitable feedback signal, there are mutually different approaches. For example, a primary-side auxiliary coil could be provided that generates, during the switch-off time of the primary-side switch, a feedback signal that delivers an indication of the output voltage. Alternatively, but also directly, the output voltage could be fed back via an optocoupler. Alternatively, other signal transmitters could also be used, in order to signal the output voltage on the secondary side directly to the control circuit on the primary side.
These feedback variants have the disadvantage that they must use an auxiliary coil, an optocoupler, or a signal transmitter. For example, in the case of an auxiliary coil in the transformer, this is comparatively complex and expensive.